1. Field of the Invention
This invention relates to composite materials which are suitable for fabricating plastic retort containers of a rigid type. More particularly, it relates to composite materials for retort containers which have good gas and vapor barrier properties without a lowering of these properties after retort treatment. The present invention also relates to top materials which are adapted for stopping or closing openings of plastic retort containers of a rigid type, especially those made of the above composite materials, and more particularly, to top materials for retort containers which do not loosen when subjected to a retort treatment or process after heat-sealing the material to an opening of a container and which have good gas and vapor barrier properties.
2. Description of the Prior Art
Known composite materials used to make plastic retort containers of a rigid type are those materials including ethylene-vinyl alcohol copolymer films laminated with polypropylene films on opposite sides, polyvinylidene films laminated with polypropylene films on opposite sides, and the like. However, these composite materials have the problem that when they are subjected to retorting, gas and vapor barrier properties are considerably lowered. With the former composite materials using ethylene-vinyl alcohol copolymer and polypropylene, the oxygen gas permeability prior to the retort treatment is approximately 0.5 cc/m.sup.2 -24 hrs-atm. When retorted, the ethylenevinyl alcohol copolymer absorbs water and the oxygen gas permeability after the retort treatment is in the order of 18 to 24 cc/m.sup.2 -24 hrs-atm. The latter polyvinylidene chloride and polypropylene composite material has an oxygen gas permeability of 4 cc/m.sup.2 -24 hrs-atm prior to the retort treatment and 8 cc/m.sup.2 -24 hrs-atm after the retort treatment.
Accordingly, when the rigid containers for retort using these materials are filled with foods, tightly sealed and subjected to retort treatment, the gas and vapor barrier properties of the containers considerably deteriorate by the retort treatment. This may cause gases such as oxygen to intrude into the container and the water in the food to be dissipated outside the container on the market, with the fear that the food will change in quality.
On the other hand, currently employed top materials for closing an opening of a plastic retort container of a rigid type include laminates comprised of an ethylene-vinyl alcohol polymer film or a polyvinylidene chloride film laminated with a simultaneously biaxially stretched film of nylon 6 on opposite sides, and a heat-sealable plastic film, such as polypropylene, formed on one of the nylon film layers.
However, this type of top material is not balanced in thermal shrinkage along the lengthwise and breadthwise directions since the two simultaneously biaxially stretched film layers of nylon 6 are formed. When the container is closed or sealed and then subjected to retort treatment, it may become loosened either in the lengthwise direction or in the breadthwise direction, not making a completely smooth surface of the top material. In addition, the above top material considerably lowers in gas and vapor barrier properties when retorted. Accordingly, when a food is filled in a retort container which is subsequently closed with the top material and subjected to retort treatment, there is the high possibility on the market that gases such as oxygen enter into the container, or the water in the food is dissipated outside the container, thereby changing the quality of the food. Moreover, since the top material is provided with two simultaneously biaxially stretched film layers of nylon 6, it is expensive.
Another top material has been proposed, which comprises an ethylene-vinyl alcohol copolymer film or polyvinylidene chloride film, a biaxially stretched film of nylon 66 formed on one side of the first-mentioned film, and a heat-sealable plastic film formed on the other side. This top material has a better heat-shrinking balance along the lengthwise and breadthwise directions than the above top material. Additionally, the nylon layer is only one, so that the top material is more inexpensive although the nylon 66 itself is relatively expensive. However, the problem of the lowering in gas and vapor barrier properties as caused by the retort treatment has not yet been solved.